Space-Time Coordinate Metadata for the Virtual Observatory

Abbreviation: STC

General Information

The Space-Time Coordinate (STC) metadata for the Virtual Observatory provides a standard for users to specify the spatial coordinates they work in; these will, for most astronomical users, be some flavor of equatorial coordinates. However, there are many variations, not only in terms of different equatorial systems, either from historical collections (FK1-4) or in the uses of Galactic or ecliptic coordinates, but also geographic, barycentric, planetocentric, and instrumental detector coordinates, most of them in spherical as well as Cartesian form. In addition, high-accuracy requirements and special situations such as spacecraft-based observatories create the need for specifying the origin of such coordinate frames – in most cases the location of the observatory. The same is true for time and spectral coordinates: for many applications it may not matter, but there are situations where it is crucial to know what timescale was used, where time was measured, or what inertial standard of rest was used to express the frequency.
What this amounts to is that for spatial coordinates it is necessary to know the coordinate system (its type and orientation) and the origin that were used, for time the timescale (UTC, TT, TAI, TDB, etc.) and the spatial reference position, for the spectral coordinate the origin in phase space, and for redshifts (Doppler velocity) the definition as well as the phase space origin. This standard explains the various components, highlights some implementation considerations, presents a complete set of UML diagrams, and discusses the relation between STC and certain other parts of the Data Model. Two serializations are described in this standard document: XML Schema (STC-X) and String (STC-S); the former is an integral part of this Recommendation.